1. Field of the Invention
This invention relates to a fuel cell system.
2. Description of the Related Art
Fuel cell systems that use fuel cells as an energy supply source have been developed in recent years. In these fuel cell systems, a reducing agent gas such as hydrogen gas is supplied to the anodes of the fuel cells, an oxidizing agent gas such as air containing oxygen is supplied to the cathodes of the fuel cells, and electrical energy composed of direct current power is generated by allowing an oxidation-reduction reaction (electrochemical reaction) to proceed between the hydrogen and oxygen in the fuel cells. A technology is employed in some fuel cell systems for circulating hydrogen gas not used in the electrochemical reaction in the fuel cells and reusing that gas in the fuel cells.
In addition, in the fuel cell system of a vehicle, when normal operation consisting of the generation of electrical power by the fuel cell is stopped in a fuel cell system during idling at the time of the vehicle stopping or while the vehicle is operating under a light load, the fuel cell is not subjected to an electrical load and further takes on a high electrical potential condition in which the electrical potential becomes higher due to the presence of residual oxygen and hydrogen, and if this state is maintained, the durability of the catalyst in the fuel cell is reduced and the performance of the fuel cell decreases.
Consequently, Japanese Patent Application Laid-open No. 2008-293708, for example, describes controlling the cell voltage of a fuel cell under a low load such as no load so as to decrease it to a predetermined voltage by increasing the hydrogen supply pressure. As a result, the decrease in performance of the fuel cell under no load or idling load can be prevented.
In the fuel cell system described in Japanese Patent Application Laid-open No. 2008-293708, the hydrogen pressure of the hydrogen supplied from a hydrogen storage tank is increased when the cell voltage of the fuel cell is lowered under a low load such as no load. Further, the cell voltage is decreased as a result of generating electrical power with residual oxygen present in the fuel cell and the supplied hydrogen. Since the generated electrical power is surplus electrical power, it is stored in the electrical storage device provided in the fuel cell system.
However, a state may occur in which the capacity of the electrical storage device is full and the electrical power generated in the fuel cell can no longer be received therein. In such a state, the surplus electrical power has to be consumed by, for example, converting the electrical power generated in the fuel cell to thermal energy. Thus, the problem of energy loss occurs.